1. Field of the Invention
This invention relates generally to a level sensing apparatus, and more particularly to an apparatus for sensing when a table top or other surface is displaced from a horizontal position and for returning the table top or other surface back to a horizontal position.
2. Description of the Background of the Invention
The surface of a surgical table must have the capability to traverse a wide range of angles in order to meet the varied demands of today's advanced surgery. It is also common to have portions of the surgical table be set at different angles for various procedures. Concern for precision, safety, reliability, maintainability, and patient comfort place stringent demands on the ability for a surgical table to traverse the necessary angles and return that table to a level position. Because of the varied sensitivity and technology level of other surgical equipment in the operating room, any movement of the surface of the surgical table should be accomplished with a minimum amount of effort and disturbance to the remainder of the operating room.
It may be critical that the table have the ability to maintain a horizontal position under extremely tight tolerances. This is especially important, for example, when transferring a patient to or from the operating table or transporting a patient within the hospital. There are other instances which require that portions of the table be kept perfectly level, for example, during certain surgical procedures. Those and other applications impose requirements such that the level along the horizontal plane must be maintained to within plus or minus one degree. The table may move under impact or vibrations, therefore the ability to automatically return the table surface to a horizontal level may be required.
Any device which purports to keep a surgical table level must be compact and must be mountable underneath the table or along its side in order to be non-intrusive to the operating staff or the patient. Such a device must not drift or otherwise fall out of calibration once set. If electrically controlled, such a device must use only a minimal amount of energy so as to suppress any electrical noise to avoid interference with delicate surgical equipment which may be present. The ability to quickly and effectively return the surgical table to a horizontally level position is necessary. Finally, such a device must be affordable.
Mechanical cranks are sometimes used to set the angle of a surgical table. Those mechanical cranks typically can be turned to set the table at any angle and subsequently return the table to horizontal. Mechanical cranks pose several problems. There is a significant effort required on the part of the operating room personnel to adjust the level of the table if a mechanical crank is used. A mechanical crank may take up valuable space or may be inconveniently located. More importantly, mechanical cranks are often subject to drifting under strong vibrations or impacts.
Electronically controlled actuators are also available for adjusting the level of the table. A bubble in a liquid filled tube may be used to determine the level of the table relative to the horizontal plane, but is hard to calibrate to within tight tolerances as sighting a bubble is somewhat subjective. While electronically controlled actuators may be more effective than mechanical cranks in some applications, level sensing is still somewhat subjective if bubble sights are used.
Inclinometers of the type used on aircraft are available and are sometimes employed on surgical tables. For example, inclinometers are used in some cardiology units. Those inclinometers employ a capacitance detection circuit to detect when the table is displaced from the horizontal. However, such inclinometers are prohibitively expensive for general surgical applications.
Commercial tilt switches are also available for detecting the horizontal displacement of a table, but are unsuitable for purposes of level sensing a surgical table because the tolerances are typically no more sensitive than plus or minus five degrees or greater.
There is a mercury switch for an automatic cutoff device disclosed in U.S. Pat. No. 3,259,202 issued to Griffeth. The switches shown in the Griffeth patent are disposed in pairs and both switches of each pair can be displaced in opposite directions off of the horizontal to create a dead band. A dead band is defined as a range through which the switches may traverse wherein neither switch is closed. The device disclosed in the Griffeth patent is described as an automotive ignition cut-off switch, but the specification indicates it may be used as a leveling device. There are several problems with the Griffeth device which make it unsuitable for use with a surgical table or other device requiring precision maneuverability. There is only a crude adjustment means to create a dead band wherein neither mercury switch is closed. Therefore, that device could not be used in applications requiring tight tolerances. Furthermore, mercury must be handled extremely carefully, especially in a sterile environment such as an operating room. Because the unit is not encapsulated, it may lose adjustment due to routine shocks and vibrations. Lastly, the Griffeth package is large and the device shown does not appear to be mountable on a side of, or underneath, a table.
U.S. Pat. No. 2,296,053, issued to Porter, discloses a shut down device which includes a pair of switches side by side, each responsive to displacement in one direction about a transverse axis. The switches employ metal balls which, when positioned in a small opening, close an otherwise open circuit. The switches may be displaced from the horizontal to a desired angle in order to create a dead band wherein neither switch is closed. There are several problems with the apparatus of Porter which make it unsuitable. The use of metal balls is crude such that the dead band cannot be finely adjusted and therefore could not be used in tight tolerance applications. Only one end of the ball sense switch is fixed which may cause erratic operation at the large angles traversed by a surgical table. Because the unit is not encapsulated, it may lose adjustment due to routine shocks and vibrations. Lastly, the Porter package shown appears to be too large to be mounted on a side of, or underneath, a table.
Another leveling device which employs a mercury switch is disclosed in British Patent No. 793,173. An electrical contact is positioned on each side of the mercury switch allowing it to close when tilted in either direction. The mercury switch may be curved and mounted such as to create a dead band wherein neither side of the switch is closed. No means for mounting the switch or sealing the switch once mounted are suggested.